One of the most challenging tasks presently facing the communications industry is providing reliable data and voice transmission over an HF ionospheric channel. The characteristics of the channel itself, which vary with both time and external conditions that affect the ionospheric propagation medium, as well as the presence of both natural and man-made interference, have made the HF channel relatively unreliable for analog voice communications. In addition, the multipath and rapid phase variations of such a channel make its use for reliable digital communications particularly difficult.
A number of proposals for improving HF communication reliability have involved the proper selection of frequencies which provide favorable propagation conditions for a particular transmission. Still, even though such "frequency management" approaches offer improvement, they usually achieve increased reliability by occupying additional spectral space, a commodity that is already very precious within the limited HF band. Other proposals that have sought to avoid the bandwidth crowding problem of frequency allocation have employed time-path diversity. The drawback with the latter approach has been the need for instantaneous status relative to the connectivity of the network.